1. Field of the Invention
The present invention generally relates to a pulse arc welding method. In particular, the present invention relates to a pulse arc welding method that enables stable welding, even when the mixture ratio of shielding gas varies and the distance between the power supply tip and the base metal changes.
2. Description of the Related Art
FIG. 9 shows the configuration of a typical consumable electrode arc welding system utilizing a welding robot. The welding system includes components described below.
The conventional welding system is provided with a welding power source PS. As seen from the figure, the power source PS receives a welding condition signal Wc as an input from a robot controller RC, and then outputs welding voltage Vw and welding current Iw for arc generation. The power source PS also outputs a feed control signal Fc to a wire feeding motor WM for performing controlled feeding of a welding wire 1. The welding condition signal Wc from the robot controller RC includes a welding voltage setting signal and a welding current average setting signal, for example. The welding voltage setting signal is provided for setting the average of the welding voltage Vw, to serve as a signal for setting the length of an arc, as will be described later. The welding current average setting signal is converted to a wire feeding rate setting signal by the welding power source PS, for setting the wire feeding rate of the welding wire 1.
The welding wire 1 is fed through a welding torch 4 as the wire feeding motor WM drives feeding rollers 41. The welding wire 1 receives electric power via a power supply tip (contact tip) 4a for ignition of an arc 3. The welding torch 4 is supported by an unillustrated robot and can be moved in accordance with operation programs stored in the robot controller RC. Shielding gas is ejected from the tip of the welding torch 4 to shield the arc 3 and the welding spot on the base metal 2 from the surrounding air. In FIG. 9, the distance between the pointed end of the power supply tip 4a and the base metal 2 is depicted as a “tip-base distance” Lw (mm), the length of the arc 3 as an “arc length” La (mm), and the distance between the pointed end of the power supply tip 4a and the free end (the lower end in FIG. 9) of the welding wire 1 as a “wire extension” Lx (mm). As readily seen, these three quantities satisfy the relation Lw=Lx+La.
It is known that there is a standard value (reference value) for the tip-base distance Lw, depending on the average of a welding current Iw. For instance, the reference value is mm when the average of the welding current Iw is not greater than 200 A. Likewise, the reference value is 20 mm when the average is in a range of 200 through 300 A, and 25 mm when the average is not smaller than 300 A. In actual welding, a fine adjustment is made to the predetermined reference value of the tip-base distance Lw in accordance with the shape of a groove in the base metal. A tip-base distance Lw which is unduly smaller or greater than the reference value will cause unstable welding.
FIG. 10 is a current-voltage waveform chart in consumable electrode pulse arc welding. In the figure, (A) shows the welding current Iw whereas (B) shows the welding voltage Vw. From time t1 through time t2, i.e. during a peak rise time Tup, a transition current, rising from a base current Ib to a peak current Ip, is caused to flow, while a transition voltage, rising from a base voltage Vb to a peak voltage Vp, is applied across the power supply tip and the base metal. From time t2 through time t3, i.e. during a peak time Tp, a peak current Ip which is not smaller than a critical current value is caused to flow with a peak voltage Vp. From time t3 through time t4, i.e. during a peak fall time Tdw, a transition current, falling from the peak current Ip to the base current Ib, is caused to flow with a transition voltage falling from the peak voltage Vp to the base voltage Vb. From time t4 through time t5, i.e. during a base time Tb, a base current Ib, which is so small that a droplet will not grow on the welding wire, is caused to flow with a base voltage Vb. The total time period from time t1 through time t5 corresponds to one pulse cycle Tf.
The peak rise time Tup and the peak fall time Tdw are set to appropriate values in accordance with the material of the base metal. In pulse MAG welding in which the base metal is steel, the times Tup and Tdw are set to small values, so the peak current waveform is substantially square. On the other hand, in pulse MIG welding in which the base metal is aluminum, the times Tup and Tdw are set to large values, so the peak current waveform is trapezoidal. For improved welding performance, the rising/falling of the transition current may be performed in various patterns not only in a linear increase/decrease pattern but also in curved patterns (see JP-A-2005-28383 and JP-A-2006-75890, for example). Also, the peak current Ip may be increased in a stepped pattern (see JP-A-2005-118872, for example). The shielding gas may be provided by a mixture of 80% argon gas and 20% carbon dioxide gas for pulse MAG welding, while 100% argon gas may be used for pulse MIG welding.
In consumable electrode arc welding, it is important to keep the arc length at an appropriate value in order to achieve good welding quality. Since the average Vav of the welding voltage Vw is generally in proportion to the arc length, the above arc length control can be performed by controlling the output of the welding power source so that the welding voltage average Vav will be equal to a predetermined welding voltage setting value. Likewise, in pulse arc welding, the welding power source output is controlled by varying the pulse cycle Tf (frequency modulation control) so that the welding voltage average Vav will be equal to the welding voltage setting value. Another method of controlling the welding power source output is to fix the pulse cycle Tf as a constant, and vary the peak time Tp (pulse-width modulation control). The welding voltage average Vav used for the control is provided by the value obtained by smoothening the welding voltage Vw.
FIG. 11 is a 1 pulse-1 droplet transfer zone chart, which shows how the specific values for the peak time Tp and the peak current Ip are to be set. In the figure, the horizontal axis indicates the peak time Tp (ms), while the vertical axis indicates the peak current Ip (A). The hatching indicates a zone of conditions to achieve a single droplet transfer in synchronization with the pulse cycle Tf (1 pulse-1 droplet transfer). When a set of conditions (called unit pulse conditions) of the peak time Tp and the peak current Ip fall within the hatched zone, 1 pulse-1 droplet transfer takes place. In actual welding, the unit pulse conditions are selected so that they are to fall within the 1 pulse-1 droplet transfer zone and also within a range by which a good bead shape (i.e. with proper bead appearance and no undercut) will result. When the peak current Ip is not constant, it is integrated over the peak time Tp, and the integrated current value is to fall within a certain range corresponding to the hatched zone. The unit pulse conditions may need to be set for individual cases since the 1 pulse-1 droplet transfer zone varies depending on various factors such as the kind of a welding wire, the shielding gas mixture ratio, and the wire feeding rate.
FIG. 12 illustrates the spot where an arc is generated, with the unit pulse conditions falling in a range that enables 1 pulse-1 droplet transfer. Specifically, an arc is ignited between the welding wire 1 (fed from the tip of welding torch 4) and the base metal 2, and a molten pool 2a is formed in the base metal 2. An arc anode point 3a is in an upper portion of a droplet 1a on the wire tip. Thus, the droplet 1a is located in an envelope of the arc 3. An arc cathode point 3b, on the other hand, is on the molten pool 2a. The droplet 1b is separated from the welding wire 1 and transferred to the base metal 2 immediately after the application of the peak current Ip is over.
When the shielding gas mixture ratio is a standard value, the unit pulse conditions are selected so as to fall within the 1 pulse-1 droplet transfer zone and to provide a good bead shape, as noted above. In pulse MAG welding (the base metal is steel), for example, the shielding gas is a mixture of argon gas and carbon dioxide gas. In Japan, a typical mixture ratio of argon to carbon dioxide is 80% to 20%.
The shielding gas may be supplied in various ways. For example, the two kinds of gases may be premixed accurately to the predetermined ratio and filled into a gas cylinder, from which the shielding gas is supplied. In this case, there is virtually no fluctuations in the shielding gas mixture ratio, and the welding is performed with the fixed ratio. However, in a large-scale factory, argon gas and carbon dioxide gas may initially be stored in separate tanks, and mixed later to a required ratio by using a mixer. Thereafter, the mixed gas (shielding gas) is distributed to individual welding machines through a centralized piping system. In such a case, there is usually a certain period of time (in starting the day's operation early in the morning, for example) when the shielding gas mixture ratio is in initial fluctuation before it settles. The range of fluctuation, which may differ among shielding gas supply systems, can be large, such as ±5% through ±10%. In addition to the initial fluctuation, there may also be fluctuations during normal factory operation. The fluctuation range in this case may be smaller than that of the initial fluctuation, but it can still be about ±5%. In another case, the welding may be performed with the shielding gas mixture ratio being adjusted to a proper value, depending on the shape of work, the required welding quality, etc. In such high-quality welding, the shielding gas mixture ratio is preset by increasing or decreasing the percentage of the argon gas. For example, use may be made of a mixture of 90% argon gas and 10% carbon dioxide gas, or a mixture of 70% argon gas and 30% carbon dioxide gas.
When the shielding gas mixture ratio is changed by increasing the percentage of the argon gas, the arc may still remain undisturbed. This is because the droplet transfer is facilitated by increasing the ratio of the argon gas. Thus, it is often unnecessary to reset the unit pulse condition even when the argon gas ratio is changed to increase.
On the other hand, when the argon gas ratio in the shielding gas is changed to decrease, the arc becomes unstable because the droplet transfer will not readily occur, as described below with reference to FIG. 13.
In FIG. 13, the generation of an arc is illustrated as three stages (A)-(C), in a case where the argon gas ratio in the shielding gas is reduced below the standard ratio. A droplet transfer proceeds in the order indicated by stages (A), (B) and (C). Specifically, with the argon gas ratio reduced, an arc anode point 3a is located at a lower end of the droplet 1a (stage (A)). In this state, the temperature at and around the arc anode point 3a becomes extremely high. Hence, metal vapor 5 blasts downward from the lower end of the droplet 1a (stage (B)), and as a reaction, the droplet 1a on the welding wire receives an upward pushing force 6 from the metal vapor 5, which makes the state of the droplet 1a unstable. Then, with the upward pushing force 6 preventing a droplet transfer (in particular, 1 pulse-1 droplet transfer) from occurring, the droplet 1a grows unduly big and spatters in a great amount, not only downward from the wire but also in undesired directions, as indicated by numeral 7 (stage (C)).
One way to address the above-described problem may be to move the arc anode point 3a from the lower end of the droplet 1a to an upper portion thereof, by increasing the peak current Ip. In this manner, though the arc anode point 3a is moved upward to an upper portion of the droplet 1a, the arc 3 will unduly flare and the arc force will increase. Accordingly, a number of undercuts will be produced, and the bead appearance will deteriorate. In addition, the increased arc force will produce more spatters from the molten pool.
The above-described problem arises when the shielding gas mixture ratio varies. Another problem comes up when the tip-base distance is changed from the reference value. As noted above, the unit pulse conditions are so selected as to fall within a predetermined range for ensuring proper droplet transfer, good bead appearance and so forth, on the premise that the tip-base distance is kept at the reference value. In such an instance, actually there may be a small deviation in the tip-base distance from the reference value. However, when the deviation is about ±3 mm or less, the welding condition does not significantly deteriorate, and thus the unit pulse condition does not need to be readjusted. However, the deviation is over ±3 mm (for example, about ±5 mm), the welding condition will unduly deteriorate.